The most common and most likely answer to this crossword clue is the 12 letter word OVERWHELMING
I've seen this crossword clue in The Guardian
O
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The cryptic/wordplay part
In cryptic crossword there's often an wordplay/cryptic part as well.This part involves a more complex hint, such as an anagram, homophone, hidden word, etc.
Sadly, We don't have an explanation for this specific crossword clue yet.
What does OVERWHELMING mean?
OVERWHELMING verb- To engulf, surge over and submerge.
- To overpower, crush.
- To overpower emotionally.
- To cause to surround, to cover.
- A situation of being overwhelmed.
- Overpowering, staggering, or irresistibly strong.
- Very great or intense.
- Extreme.